This invention relates generally to enhanced tubes and more particularly to porous coated enhanced tubes and a method of enhancing evaporator tubes.
In an evaporator of certain refrigeration systems a fluid to be cooled is passed through heat transfer tubing while refrigerant in contact with the exterior of the tubing changes state from a liquid to a vapor by absorbing heat from the fluid within the tubing. The external and internal configuration of the tubing are important in determining the overall heat transfer characteristics of the tubing. For example, it is known that one of the most effective ways of transferring heat from the fluid within the tube to the boiling refrigerant surrounding the tube is through the mechanism of nucleate boiling.
It has been theorized that the provision of vapor entrapment sites or cavities on a heat transfer surface cause nucleate boiling. According to this theory the vapor trapped in the cavities forms the nucleus of a bubble, at or slightly above the saturation temperature, and the bubble increases in volume as heat is added until surface tension is overcome and a vapor bubble breaks free from the heat transfer surface. As the vapor bubble leaves the heat transfer surface, liquid enters the vacated volume trapping the remaining vapor and another bubble is formed. The continual bubble formation together with the convection effect of the bubbles traveling through and mixing the boundary layer of superheated refrigerant, which covers the vapor entrapment sites, results in improved heat transfer. A heat exchange surface having a number of discrete artificial nucleation sites is disclosed in U.S. Pat. No. 3,301,314.
It is known that a vapor entrapment site or cavity produces stable bubble columns when it is of the re-entrant type. In this context, a re-entrant vapor entrapment site is defined as a cavity in which the size of the surface pore is smaller than the subsurface cavity. Heat transfer tubes having re-entrant type pores are disclosed in U.S. Pat. Nos. 3,696,861 and 3,768,290.
It has been discovered that an excessive influx of liquid from the surroundings can flood or deactivate a re-entrant type vapor entrapment site. However, a heat transfer surface having subsurface channels communicating with the surroundings through surface openings or pores have been found to provide good heat transfer and prevent flooding of the vapor entrapment site.
As disclosed in U.S. Pat. No. 4,438,807 assigned to the present assignee, an internally and externally enhanced heat transfer tube, having an internal rib and an external helical fin (creating a subsurface channel) communicating with the surrounding liquid through surface openings (pores) is manufactured by a single pass process with a tube finning and rolling machine. According to the disclosed process a grooved mandrel is placed inside an unformed tube and a tool arbor having a tool gang thereon is rolled over the external surface of the tube. The unformed tube is pressed against the mandrel to form at least one internal rib on the internal surface of the tube. Simultaneously, at least one external fin convolution is formed on the external surface of the tube by finning discs on the tool gang. The external fin convolutions form subsurface channels therebetween. The external fin convolutions also have depressed sections above the internal rib where the tube is forced into the grooves of the mandrel to form the rib. A smooth roller-like disc on the tool arbor is rolled over the external surface of the tube after the external fin convolution is formed. The smooth roller-like disc is designed to bend over the tip portion of the external fin so that it touches the adjacent fin convolution and forms an enclosed subsurface channel. However, the tip portion of the depressed sections of the external fin, which are located above the internal rib, are also bent over but do not touch the adjacent convolutions, thereby forming pores which provide fluid communication between the fluid surrounding the tube and the subsurface channels. However, this method of enhancing tubes does not lend itself to enhancing hard tubes, such as titanium.
As disclosed in U.S. Pat. No. 4,129,181 an externally enhanced heat transfer tube is manufactured by applying a very porous reticulated organic foam layer in contact with the tube surface, and then plating a thin metal coating on the foam substrate. The foam layer is then pyrolyzed in the range of 575.degree.-900.degree. F. which can result in degradation of the mechanical properties of the base tube by annealing the tube.
As disclosed in U.S. Pat. No. 3,990,862 an externally enhanced heat transfer tube is manufactured by spraying of metallizing of metallic powders to a metallic substrate using a single spray nozzle in which the oxidizer-fuel gas balance is of prime importance.
Thus, there is a clear need for a method and apparatus for applying a porous coating to an evaporator tube that would, to a large extent, overcome the inadequacies that have characterized the prior art.